JavaScript中实现接口继承

JavaScript没有原生的继承机制，但是可以模拟实现。其中的关键是使用Function.apply()或Function.call()。自己实现了这种模拟过程，在这里记一笔。

var Utility = {
	trace: function(s){
		WScript.Echo(s);
	},

	getDateTime: function(){
		var d = new Date();
		var s = new String();

		s = d.getFullYear() + "-" + (d.getMonth() + 1) + "-" + d.getDate() + " " + d.getHours() + ":" + d.getMinutes() + ":" + d.getSeconds();

		return s;
	}
};

var Class = {
	Inherit: function(child, parent){
		for(var p in parent)
		{
			var lambda = parent[p];
			if(lambda instanceof Function)
			{
				child[p] = lambda.bind(child);
			}
		}
	}
};

Function.prototype.bind = function(obj){
	var method = this;
	return function(){
		return method.apply(obj, arguments);
	};
};

//BaseLogWriter不能被直接使用，只有子类继承之，并实现Write()方法后，在子类中Debug等方法才可用
var BaseLogWriter = {
	Debug: function(s)
	{
		var strLog = Utility.getDateTime() + " [Debug] " + s;
		this.Write(strLog);
	},

	Info: function(s)
	{
		var strLog = Utility.getDateTime() + " [Info] " + s;
		this.Write(strLog);
	}
};

function ConsoleLogWriter()
{
	// 继承BaseLogWriter的方法
	Class.Inherit(this, BaseLogWriter);

	this.Init = function()
	{
	};

	this.Final = function()
	{
	};

	this.Write = function(s)
	{
		WScript.Echo(s);
	};
}

function TextLogWriter(strLogFile)
{
	// 继承BaseLogWriter的方法
	Class.Inherit(this, BaseLogWriter);

	this.privs = {
		logFile: strLogFile,
		fso: null,			// FileSystemObject
		text: null			// TextStream
	};

	this.Init = function()
	{
		this.Final();

		this.privs.fso = new ActiveXObject("Scripting.FileSystemObject");
		this.privs.text = this.privs.fso.OpenTextFile(this.privs.logFile, 8, true);
	};

	this.Final = function()
	{
		if(this.privs.text != null)
		{
			this.privs.text.Close();
			this.privs.text = null;
		}

		if(this.privs.fso != null)
		{
			this.privs.fso = null;
		}
	};

	this.Write = function(strContent)
	{
		this.privs.text.WriteLine(strContent);
	};
}

function main(){
	var cWriter = new ConsoleLogWriter();
	cWriter.Init();
	cWriter.Debug("test console log 1");
	cWriter.Info("test console log 2");

	var tWriter = new TextLogWriter("log.txt");
	tWriter.Init();
	tWriter.Debug("test text log 1");
	tWriter.Info("test text log 2");

	cWriter.Final();
	tWriter.Final();
};

main();

上面这些JS代码保存至Interface.js，在windows控制台下运行cscript.exe //Nologo Interface.js，即可分别在打印和log.txt中见到运行结果:

 控制台打印出：

　　2011-4-5 21:13:32 [Debug] test console log 1

　　2011-4-5 21:13:32 [Info] test console log 2

log.txt里记录是：

　　2011-4-5 21:13:32 [Debug] test text log 1

　　2011-4-5 21:13:32 [Info] test text log 2

可见实际Debug和Info里调用的Write()是各个子类中的Write()。子类中Write()实现的不同，那么Debug()和Info()的行为就不同，这和C++中多态机制类似，可见这里模拟出的继承机制是成功的。